1. Field of Invention
The present invention relates to a self-reference interference wavefront sensing technology, and more particularly to a circular common-path point diffraction interference wavefront sensor. It belongs to the optical detection technology field.
2. Description of Related Arts
It is always needed for the real-time diagnosis of the distortion wavefront, the surface detection of the optical components, the laser beam purification in the laser adaptive optical system and the high-power laser system to measure the wavefront phase distribution of the light wave. The commonly used wavefront sensing technology comprises the Shack-Hartmann wavefront sensing technology, the curvature wavefront sensing technology and the self-reference interference wavefront sensing technology. The basic principle of the Shack-Hartmann wavefront sensor is that the incident wavefront is divided into several sub-wavefronts using the microlens array, and then the centroid offset of the focused spot of every sub-wavefront obtained via respective micro-lens is measured by the two-dimensional array photodetector for obtaining the slope of wavefront under test, and finally the phase distribution of the testing wavefront is obtained by wavefront reconstruction algorithm. The Shack-Hartmann wavefront sensor has the convenient calibration and simple structure. Its wavefront recovery process can be completed through the linear matrix operations. Therefore, it can achieve the real-time measurement and is a widely used wavefront sensor. However, the measurement accuracy of the Shack-Hartmann wavefront sensor is limited by the spatial resolution thereof. Compared with the Shack-Hartmann wavefront sensor, the curvature wavefront sensing technology has some important advantages as below. The wavefront curvature distribution signal obtained by the curvature wavefront sensor can be directly used to control the wavefront correction system for correcting the distortion wavefront of testing laser beam without the complex operations in the Shack-Hartmann wavefront sensor, thereby quickening the feedback speed. However, the curvature wavefront sensor is only adapted for detecting the lower spatial frequency of the distortion wavefront, and the accuracy of the curvature wavefront sensor is lower than that of the Shack-Hartmann wavefront sensor for the higher spatial frequency of the distortion wavefront.
The self-reference interferometer, as the wavefront measurement technology, has played an important role in the optical system and the laser beam characterization field. The point diffraction interferometer (PDI) is a common-path interferometer with the simple structure, which is firstly proposed by R. N. Smartt in 1972. The basic principle of the PDI is shown in FIG. 1. A small light-through pinhole with the proper size is placed at a semitransparent plate, thereby forming a pinhole mask which is placed at the focal plane of the convergent lens. When the diameter of the small light-through pinhole is small enough, the reference wave which is approximately considered as the ideal spherical wave can be formed by the pinhole diffraction. The wavefront through the semitransparent plate contains the phase information of the testing wavefront. By analyzing the interference fringes produced by the reference wave and the signal wave, the phase distribution of wavefront under test can be reconstructed. However, the reference wave and the signal wave of the conventional point diffraction interferometer almost keep the same geometrically optical axis, and produce the interferogram which generally includes the few fringes, thus the Fourier analytics cannot be used to extract the phase information of wavefront under test. Also, due to the common-path structure, it is difficult for the conventional point diffraction interferometer to introduce the phase shift between the reference wave and the signal wave.
In 1964, M. V. R. K. Murty proposed another self-reference interferometer, namely, the cyclic radial shearing interferometry (CRSI) in the literature of “A compact radial shearing interferometer based on the law of refraction, Appl. Opt, 3(7):853-858 (1964)”, in which the wavefront under test are respectively zoomed in and zoomed out by the CRSI, and then produce the interference within the superposition area thereof, so that the phase distribution of the distortion wavefront is extracted from the interference fringe. Due to the common-path structure without the special reference wave, the CRSI is insensitive to the environmental vibration. Therefore, the CRSI can be applied under the bad operational environment, and especially, the spatial phase modulation technology is introduced, thus it is easy to extract the phase information of the testing wavefront, namely, the overall detection of the distortion wavefront can be achieved by a single interferogram, and has the higher accuracy, so the CRSI has the great advantage in the transient wavefront detection field. A. R. Barnes and L. C. Smith disclosed that the output wavefront of the near field and the far field for the large aperture laser system is detected based on the CRSI in the literature of “A combined phase, near and far field diagnostic for large aperture laser system, Proc. SPIE. 3492, 564-572 (1999)”. The tested wavefront of the CRSI is zoomed in and zoomed out by the telescope system, so for the slowly varying wavefront, when the amplification of the telescope system is large enough, the expanded wavefront can be considered as the ideal plane wave. When the phase distribution of the tested wavefront is complex, even the amplification of the telescope system is increased; it is difficult to obtain the ideal reference plane wave. Therefore, the wavefront phase extracted from the interference fringes reflects essentially the phase difference between the zoomed-in wavefront and the zoomed-out wavefront instead of the actual phase distribution. To obtain the actual phase distribution of the testing wavefront, Tsuguo Kohno et al disclosed the wavefront phase iterative algorithm for reducing the error in the literature of “Radial shearing interferometer for in-process measurement of diamond turning, Opt. Eng. 39(10):2696-2699 (2000)”. Subsequently, combined with the condition that the zoomed-in wavefront and the zoomed-out wavefront have the relative translation, Da-hai Li et al obtains a wavefront phase iterative algorithm which is more close to the practical applications in the literature of “Improved formula of wavefront reconstruction from a radial shearing interferogram, Opt. Lett. 33(3):210-212 (2008)”, thereby further improving the detecting accuracy of the wavefront phase. However, if the error corrections want to reach the higher accuracy by the iterative algorithm, the more sampling points of the wavefront phase must be reconfigurable computed. Accordingly, the computational complexity is very large. Therefore, the applications in the line measurement, the transient wavefront detection and other fields are limited.